The avermectin series of compounds are potent anthelmintic and antiparasitic agents against internal and external parasites. The natural product avermectins are disclosed in U.S. Pat. No. 4,310,519 to Albers-Schonberg et al., and the 22,23-dihydro-avermectin compounds are disclosed in Chabala et al., U.S. Pat. No. 4,199,569. Administration of the avermectin compounds occur orally, parenterally or topically.
Conventional topical formulations available for use, such as sprays, dips, powders, shampoos and the like do not provide acceptable extended efficacy against ectoparasites, especially against fleas and ticks. These formulations fail because the animals are readily reinfested by fleas and ticks after treatment with the above-noted conventional formulations simply by returning to a flea infested environment. Additionally, the use of shampoos, dips and spray formulations are not very convenient to apply, and have the potential to create environmental waste problems. Further, topical formulations of currently available medicinal agents have not demonstrated efficacy against endoparasites, such as heartworms and nematodes.
It is known in the pet care industry that sustained release of an insecticide is obtained by incorporation of the insecticide into a polymeric system. However, conventional polymer based formulations rely on the vaporization of the active compounds, which means this type of system may not be used for non-evaporable drugs. See U.S. Pat. Nos. 3,852,416 and 4,172,904.